In the last case they do not specify, this is what the partitive is about, but I still have some doubts about it, as when it is correct to use it or not, sometimes it is not so easy to determinate.

I thought it might be that.

The use of the partitive article: (du, de la, la, des)

-- When the adjective preceeds the noun, it is shortened to de
de belles maisons; de beaux rêves.
- When the noun is understood but not stated, the same rule
applies: Prenez ces allumettes, j'en ai d'autres
Avez-vous des poires? - Oui, j'en ai de très grosses

-- In a few instances, where adjective and noun have come to form
a single idea, it is customary to use des rather than de:
des jeunes gens; des jeunes filles; des petits enfants.
-Applying this rule to the singular: bois de ce vin; donnez-moi
de votre buvard. But it is much more often: du bon vin; de bonne
viande, rather than the strictly correct de bon vin; de bonne viande.

-- Pas un usually means not a single one: je n'ai pas un sou.
There is a special circumstance in which a negative is followed by
a full partitive: Ce n'est pas de l'or, c'est du cuivre; Ce ne sont
pas des anglais, ce sont des americains.